Communication system

ABSTRACT

A bidirectional broadband and telephony network is controllable from an exchange or head end of the network. An optical fiber (16 ) carries the telephone services to customers&#39; premises (30, 31) via passive optical splitters (17, 18). Telephony is provided on one wavelength, broadcast TV on a second wavelength, and bidirectional asynchronous time division information on another wavelength using ATD techniques. Time-slots are allocated for information packets so they can travel without interference to and from receiving stations and the round trip delay between stations is adjusted to achieve this whilst maintaining minimum bandwidth requirements. Dynamic allocation of spare capacity is provided for bursty services.

BACKGROUND

1. Field of the Invention

The invention relates to communication systems and to such systemsemploying a passive network, for example a passive optical network. Suchpassive networks control the operation of the network at the localexchange or head end for a number of customers.

2. Prior Art and Other Considerations

For telephone companies, optical fibres have the capacity to handle arange of services beyond telephony.

In the past, coaxial cable TV networks have, in the main, been usedtotally separately from the telephone network. It is now becoming clearthat beyond the development of the switched-star network and improvedadvanced optical fibre technology, there is a need for a more flexibletelephony based system with additional services provided.

SUMMARY

The present invention is concerned with a configuration capable ofproducing an enhanced service capability typically on a local fibrenetwork using passive networks employing optical splitting to reduce andshare costs.

According to the invention there is provided a bidirectional broadbandand telephony network controllable from the exchange or head end of thenetwork including means at the exchange or head end for allocating timeslots for information packets to travel without interference to and froma plurality of receiving stations, wherein the allocating means includesmeans for determining the round-trip delay between the exchange or headend and each of the receiving stations and means for adjusting the roundtrip delay to ensure correct spacing of the information packets duringpassage through the network, and includes means for interrogatingstations and enforcing allocation thereto to ensure each stationsminimum bandwidth requirements are fulfilled, and means for dynamicallyallocating spare capacity to each station for bursty services.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example with reference to theaccompanying drawings in which:-

FIG. 1 shows a network configuration capable of handling telephony andbroadband services eg broadcast TV).

FIG. 2 shows one example of the frame structure of the telephony channelof FIG. 1.

FIG. 3 shows an expanded configuration capable of handling broadbanddialogue services.

FIG. 4 shows the customer end configuration of the ATD channel.

FIG. 5 shows the customer equipment of FIG. 4 in more detail;

FIG. 6 shows the exchange end block 40 in more detail;

FIG. 7 shows the `d` value controller of FIG. 6 in more detail;

FIG. 8 shows a modified frame structure of the telephony channel toaccommodate the ATD control information; and

FIG. 9 shows the cell structure of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a proposed configuration for a passive optical network. Theexchange telephony block 10 provides telephony at a rate typically 20Mbit/s on one wavelength (λ1) and the TV block 11 provides broadcast TVon a second wavelength (λ2) and these pass to the local exchange end 15.

A single mode optical fibre (16) is fanned out to a number of customerspremises (30, 31) via passive optical splitters (17, 18) located at thecabinet (c) and street Distribution Point (DP) positions respectively,for example. An 8-way split is allowable at the cabinet and a 16-waysplit at the DP. However, the maximum amount of splitting is selected tobe 32 to keep within the optical power budget available for broadbandbit-rates in this particular configuration.

For customers requiring telephony only, current system design views arefavoring a network which features a maximum optical split up to 128 waysand a highest bitrate of operation of 20 Mbit/s. This bitrate/splitcombination allows an attractive set of options for both business andresidential customers. At the maximum split of 128 (120 customers plus 8test ports), capacity would be available to feed each customer, ifdesired, with an ISDN 144 bit/s channel or equivalent capacity. Forbusiness districts, where multiple line customers are in the majority, alower optical split would be employed, allowing higher capacities to bedelivered per customer.

The advantages of a passive telephony network of this kind include lowmaintenance costs, reduced amounts of installed fibre, low fibre countcable requirements and sharing of exchange opto-electronic devices.

For broadband services the total number of customers can be increased to128 by using 4 feeds from the head end to the cabinet. Each feed splits32 ways. In addition, the single telephony-only feed can be split overthe same fibre paths to all 128 customers because of its smallerbit-rate.

An example of the frame structure of the telephony channel for FIG. 1 isshown in FIG. 2.

Whilst the FIG. 1 configuration can handle telephony and broadcast TVthere is a need to provide broadband services in both directions forviewphone or high speed data handling.

Although these services could theoretically be provided (ignoring costsand other constraints) by employing separate wavelengths to and fromeach customer, however a different approach now proposed which couldprove to be less costly and also capable of realization much morereadily is to carry such services by introducing two new channels, onefor upstream (i.e. customer to local exchange) and one for downstreamconnections (local exchange to customer).

FIG. 3 shows the proposed configuration capable of handling theadditional facilities. Individual connections are multiplexed withinthese channels using ATD (Asynchronous time division) and carried usinga common new wavelength for the two directions of transmission.

To control the multiplexing of upstream information in one form of theembodiment, it is proposed to convey information via the telephonychannel to update the exchange end on the condition of queues at eachcustomer's end. The second form of the embodiment conveys allinformation on the condition of queues using the ATD channel.

Furthermore, in either form of the embodiment at the local exchange theATD channel would be routed as shown via the service access switch 42(also based on ATD) and used as a very flexible integrated accesschannel for any customer requiring a mix of broadband and narrowbandservices. Hence terminal A of the switch 42 could handle incoming andoutgoing information to/from the ATD Broadband network and terminal Bwill handle incoming and outgoing information from other networks.

The ATD channels are handled by ATD block (40) using wavelength 3 andfor both outgoing and incoming information which will pass via the localexchange end 15 in optical form to be handled with other broadband ornarrow band services. This wavelength carries two channels, one for thedownstream directions to the customers and one for the upstreamdirection from the customers end to the exchange.

All ATD packets from the exchange end are received by all customers andthe actions performed are determined by header information such that acustomer will accept the packet and/or re-use the packet for upstreamcommunication or ignore the packet.

An ATD channel bit rate of about 150 Mbit/s shared over 32 customersproduces and average capacity of about 4 Mbit/s per customer. Given thatATD dynamically assigns capacity, it is expected that customers willusually be able to obtain much higher capacities on demand (eg a highprobability of least 15 Mbit/s). Also, in either form of the embodimentalmost the entire capacity of the link can be utilized by a singlecustomer during those times when other customers are idle. Thus a grossbit rate of 150 Mbit/s for the ATD channel could be suitable for a widerange of business customers having bursty data services and narrowbandservices to transmit. In the longer term it will also be suitable forresidential customers requiring viewphone connections. At the moment itis not foreseen whether constant bit rate or variable bit rate videocodecs will be used for viewphone (the latter perhaps based on a meanbit-rate of 64 kbit/s and a peak bit rate of 384 kbit/s. However the ATDchannel does not restrict the choice since it would be capable ofcarrying either format.

Very large bit rate services (broadcast TV) are carried on separatewavelengths (i.e. on wavelength λ2). Therefore the bit rate required percustomer on the ATD channel is significantly smaller than would berequired if ATD carried a full range of services.

The proposed ATD configuration lends itself to being retrofitted to theFIG. 1 arrangement or to a telephony only passive optical network of thetype disclosed, without replacing the entire network.

In either form of the embodiment shown, the ATD channel is provided withits own specific wavelength for the upstream and downstream direction,and customers' equipments are fitted with a wavelength filter (20) toreceive this channel. Customer 30 can have the capability of multipleservices including ATD, telephony and broadcast TV. The customer 31still only needs the telephony service and therefore is so restricted.

The ATD channel carries a digital multiplex with a gross bit rate ofeither 150 Mbit/s or 600 Mbit/s for example.

The ATD channel is handled at the customer's end as shown in FIG. 4. Awavelength filter (20) is associated with each channel and atoptical/electrical interfaces (45, 46) data is buffered respectively toand from the local exchange end (via the distribution point andcabinet).

Block 47 described in more detail in FIG. 5 will contain a phased-lockedloop, locked to the ATD transmission from the local exchange end, andused to control the frequency of transmission in the upstream direction.Additionally, each block 47 will contain an elastic buffer. The localexchange end adjusts the size of this buffer so that there is an equalnumber of bits in each round-trip path (exchange end to any customer andback). Suitable signals to control the size of the elastic buffers aresent over the ATD channel to access control (47) from the local exchangeend.

The configuration of FIG. 4 is shown in more detail in FIG. 5. Thewavelength filters 20 of FIG. 4 cooperate with the downstream andupstream opto-electronic interfaces 46, 45. The electrical output ofinterface 4l is received by register/buffer block 50. The output ofblock 50 is available to interface 45. The block 50 also contains thephase locked loop locked to the ATD transmission from the local exchangeend. The elastic buffer within block 50 ensures correct adjustment ofthe round trip delay. Control block 52 connected to block 50 willcontrol upstream access as well as downstream control.

The downstream buffer 54 handles received packets passed via control 52and the data is passed to the appropriate service buffers 57, 58. Buffer57 may handle viewphone or voice information and buffer 58 may deal withbursty data for example. The received packets are then available forconversion in standard depacketizer for receipt by the customer as voiceor other data. Viewphone or data for transmission back to the exchangeend for example would be received (after conversion in to digital formas necessary and packetized) by buffers 55 and 59. Buffer 55 couldhandle viewphone or voice services for example and buffer 59 couldhandle other data. Data from these buffers can be made available fortransmission dependent on polling block 49 which will operate under thecontrol of allocation control 56. This polling and allocation willdepend on the priority given to services associated with buffers 55 and59 and the allocation allowance provided from the `d` counters withinblock 56. In one embodiment additional information concerning packetarrivals in buffers 55 and 59 (waiting to be sent) is detected inmonitoring block 48 and sent via the telephony channel (as representedby the broken line in FIG. 5 from queue monitoring block 48).

The packet information made available from buffers 61 and 62 passes toupstream packet buffer 53 to control 52 which will add information tothe header to indicate `d` allocation requirements (as described in moredetail below) and thence to register/buffer 50 to interface 45 to passalong the optical medium to the exchange end.

In the downstream direction, the local exchange end transmits acontinuous series of ATD cells, which may be full or empty. Typicallyeach customer's equipment will extract cell synchronization from eitherspecially recognizable synchronization cells injected regularly by thelocal exchange end, or from recognizing the known information contentand header content of empty cells.

In the upstream direction, successive customer transmissions may differin phase but should not differ in frequency. Therefore to ensure thelocal exchange receiver can maintain bit synchronization with respect toupstream transmissions, each cell will begin with sufficient bits sothat the receiver can synchronize to any change of phase. A suitableline code, e.g. coded mark inversion (CMI), will be used in bothdirections of transmission.

Excluding the synchronization field (which is not considered to add morethan a few per cent to the overall cell length, the cell size is around36 octets, with a format as described below. The same cell size is usedfor upstream and downstream transmissions.

The downstream ATD block at the exchange end is shown in more detail inFIG. 6. The information from the service access switch 42 is received bypacket buffer 60. Customer identification is taken care of by connectionnumber translator 61. The assembly block 67 assembles the packet in theformat required to meet cell size and synchronizing field requirementsand this is made available to the customer via the opto-electronicinterface 74. Ranging control 70 can also have access to assembler 67 todetermine and set round-trip delay. The ranging parameters can be setvia a man-machine interface for example. The connection numbertranslator 64 detects incoming ranging control cells and passes this toblock 70 to facilitate measurement of round trip delay. Ranging controlcan then if required send out further control cells via blocks 67 and 74to the customer end to adjust the size of the elastic buffer withinblock 50 to ensure the round trip delay is identical with othercustomers. Additionally translator 64 holds connection numberinformation identifying customers.

Assembler 68 provides the access and control field informationrespectfully to accompanying the packet information to identifycustomers and also identify the type of information being sent (e.g.ranging indication or normal data). Controller 66 has the stored `d`value applicable to the amount of access available and extractsinformation from the headers of packets received from the customers viablocks 73 and 63 to determine the latest `d` allocation requirements.

The controller 66 is shown in more detail in FIG. 7. The changeallocation control 76 receives the change allocation header subfield (asdescribed below) from upstream packet buffer 63. Current `d` values arestored for all customers in store 79. This is connected to poller 71 toallow loading of current `d` values to the poller or to receive from thepoller 71 a request for all current `d` values. The poller 71 alsoprovides a message to block 76 to reset rates or to initiateconfirmation of the latest received "change allocation" information froma customer. Registers 78 have information on each customer concerningrequests, modification or checks on any customer `d` value. The latestallocation requests for all customers are held in store 77.

Returning to FIG. 6, additional received information from the customersis available via opto-electronic interface 73 and this passes to packetbuffer 63. This buffer provides an output to the service access switch42. The buffer 63 also is accessed by the translator 64 which translatescustomers connection numbers.

Polling block 71 may make use of `d` allocation information provided onthe ATD channels and (in one embodiment) use the telephony channel (asrepresented by the broken lines) for polling customers who haveindicated that packets are waiting. The telephony channel of FIG. 2showed 17 subframe periods with a ranging capability within a single 2ms frame and FIG. 8 is structured so that each customer has access toseparate parts of the subframe (in this example subframe part 1) toenable both telephony and ATD users access to the telephony and ATDusers access to the telephony channel (in one of the embodiments). Asshown, the configuration is of 70 sectors each of 32 bits for ATDcontrol or telephony plus 18 bits spare to give the 2258 channels ofsize 1 bit.

The two multiple-user ATD channels on wavelength λ3 will typically havethe cell format shown in FIG. 8. Each packet 80 will be carried in thetime slot structure in the system in either direction and will includean information field portion 81 and a header portion 82.

The header is provided with three sub-fields 82a-82c relating to thesub-fields of CONTROL, ACCESS, and CONNECTION ID respectively.

The `ACCESS` sub-field 82b contains customer address information, to beused to control transmissions in the upstream direction. As thedownstream ATD cells are sent from the local exchange end, each with aspecific setting of the ACCESS field provided by block 68 of FIG. 6, acustomer's equipment with matching address has the right to transmit oneATD cell in the upstream direction. The address value is set by thelocal exchange end based on information received about active terminalsas described below. Thus this sub-field determines which customer isallowed to transmit next.

The Connector I.D. (CONN ID) 82a sub-field of the header contains aconnection number (typically 16 bits) which is examined by the receivingcircuitary of each customer's equipment (i.e. block 52) to determinewhether the downstream cells should be kept or discarded. There isadditionally a requirement to include some bits for label errordetection/correction (e.g. 4-8 bits). Thus the CONN ID sub-fielddetermines which customer receives a downstream cell.

The information field 81 has a size typically around 32 octets.

A suitable method of downstream mulitplexing can employ a time slotconfiguration described in European Patent Publication 0168265 forexample, where `d` packages can be assigned to users to preventmonopolising the network.

The control (CTRL) sub-field 82c of the header contains the followingcontrol information:

full/empty indication.

reset/transmit indication.

broadcast indication.

ranging indication.

priority indication.

change allocation.

On the ATD channel the local exchange end equipment can measure theround-trip delay to any customer and back. Such measurement periods areinitiated by the local exchange end sending specially recognised`ranging` cells, with the upstream ACCESS field (upstream right totransmit) set to the customer to be measured. A continuous train of suchcells are sent from block 70 of FIG. 6 from the exchange end throughoutthe measurement period. This mode of operation is required so that thelocal exchange end can adjust the delay in the round trip path to ensurethat upstream cells from different customers are correctly interleaved.However, ranging is considered to be only occasionally required and maybe initiated via a man-machine interface as described above in relationto FIG. 6.

UPSTREAM MULTIPLEXING (a) 1st Embodiment

In the first form of the embodiment the telephone channel is used toprovide the local exchange with information about active terminals.

Because the original telephony channel is occassionally unavailablethrough ranging operations for telephony-only customers, as shown in theexample frame structure of FIGS. 2 or 7, control of upstream polling isvia two mechanisms:

information from this telephony channel on packet arrivals

polling `recently` active users for their whole `d` value.

On this channel, see FIG. 8, (originally only for telephony) eachmultiservice customer is assigned 70 bits per frame. This is furthersubdivided into 64 bits which are set according to packet arrivals, anda further 6 bits indicating the sum of all packet arrivals in the lastframe period.

The 70 bits are dispersed regularly throughout the frame such that anyadjacent group of 32 bits on this channel carries 1 bit from each of thebroadband customers. If an ATD cell arrives for upstream transmission ona customer's equipment, this is indicated to the local exchange end bysetting one bit, i.e. that customer's next assigned bit in the currentframe. Further packet arrivals will be likewise indicated. In this way,the local exchange end receives updated information about waiting cellson each customer's equipment approximately every 1.5 microseconds (apartfrom the ranging period of the telephony channel).

(b) 2nd Embodiment

In the second form of the embodiment, which does not require the use ofthe telephony channel the upstream multiplexing is controlled from thelocal exchange and based on two mechanisms.

polling on each customer' control channel exactly once every 4milliseconds

polling all active customers at least once every 125 microseconds fortheir permitted allocation (i.e. their `d` value--see below)

The polling is effected by blocks 71 and 68 of FIG. 6. For the purposesof polling, the local exchange access controller 68 regards eachcustomer's access equipment as two devices. At the customer's end thesetwo `devices` are the separate responses of the access equipment to twodifferent ACCESS addresses which it will match with.

The first `device` is the allocation control (Control 56 and block 52 ofFIG. 5) which is responsible for sending all `change allocation`messages to the controller at the local exchange, via appropriatesettings of the header. As stated above, it has a guaranteed access rateof one cell every 4 milliseconds. User data (e.g. signallinginformation) can be included in the information field provided theheader is also set to `full`. In this case the data part is treated asbelonging to the connection whose identity is given by CONN ID.

The frequency of these transmissions from all customers' equipments maybe used to provide a minimum level of monitoring of the round-trip delayso as to compute any necessary fine tuning to be conveyed to thecustomer's equipment.

The second `device` comprises the control 52 and buffer 53 and is alwaysconsidered to be active by the controller at the local exchange, and isgiven a permitted upstream allocation--which may include zero. Changingthe allocation of this device is the method by which the customer'sequipment gains an appropriate upstream access rate for its queues.

Whenever a customer is polled on either `device` it must return either a`full` cell or `empty` cell and repeat the value in the ACCESS field.For all upstream transmissions the transmitting source is identified atthe local exchange through the setting of ACCESS.

To service waiting queues at the customer's end, any downstream cellwith either of the two matching values of ACCESS causes the next waitingcell to be transmitted upstream. This upstream cell will then be set tobe transmitted upstream. This upstream cell will then be set to `full`and the CONN ID value will be set to the appropriate connection number.In addition, if the ACCESS field matches the control channel then the`change allocation` part of the header is appropriately set.

The 8 settings of the `change allocation` field are:

increase `d` by 1/ repeat increase `d` by 1

no change/ repeat no change

decrease `d` by 1/repeat decrease `d` by 1

`d` equals zero/ repeat `d` equals zero

Whenever a `change allocation` message is sent from the customer's endthe information is repeated by the controller at the local exchange. Itis inserted in the `change allocation` field of the next poll requestsent downstream on the same ACCESS channel number via blocks 76 and 77of FIG. 7. This allows the customer's access equipment to check that theinformation it sent last time was correctly received. If there is anyerror the customer's end sends a `repeat` type setting (see the abovesettings) and records the error. (The repeat type setting is not used inthe first embodiment case).

To ensure that any repeat-type `change allocation` messages arecorrectly responded to, the local exchange controller (block 66)maintains 3 registers 78 for each customer, i.e. dREQ, dCHECK, and dMODas shown in FIG. 7. The register dREQ determines the current allocation(as described in the next section), the other two registers containintermediate values. The rules governing the use of these 3 registersare set out in Table 1.

                  TABLE 1                                                         ______________________________________                                        CHANGE ALLOCATION MESSAGES -                                                  RESPONSE AT LOCAL EXCHANGE                                                                            REGISTER CHANGES                                      CUSTOMER   FOLLOWED BY  IN LOCAL EXCHANGE                                     SENDS      (4 ms later) CONTROLLER                                            ______________________________________                                        OPERATION  OPERATION'   dCHECK first loaded into                              (eg NO     (eg INCREASE dREQ perform OP' on                                   CHANGE)    d BY 1)      dREQ and load result in                                                       dCHECK                                                OPERATION  REPEAT       perform OP' on dREQ and                                          OPERATION'   load result in dMOD                                   REPEAT     OPERATION'   dMOD first loaded into                                OPERATION               dREQ perform OP' on                                                           dREQ and load result in                                                       dCHECK                                                REPEAT     REPEAT       perform OP' on dCHECK                                 OPERATION  OPERATION'   and load result in dMOD                               ______________________________________                                    

This scheme fully corrects any isolated case of an incorrectly receivedcontrol message, in either the upstream or downstream direction. Incombination with a limited error correction field in all ATD headers(e.g. correcting any single bit error), the number of cases where theallocation is incorrectly set is expected to be extremely small.

Entry into the state where the allocation is zero is always performed bythe use of the `d` equals zero operation, rather than decrease `d` by 1.This absolute setting provides an additional safeguard against the localexchange dREQ register being in long term misalignment with the valueheld at the customer end.

As well as knowing where cells are waiting, the exchange end alsooperates a scheme which provides guaranteed access to each customerevery 125 microseconds. The exchange end block 79 stores an integervalue, known as `d` value, for each customer which is the number ofcells the customer requires to be guaranteed to be transmitted upstreamevery 125 microseconds. Every time a customer is polled by block 71 anassociated counter in block 71 is decremented, starting from the value`d`. When the counter reaches zero, the customer cannot be polled evenif he has further cells waiting. Polling of that customer can only beginagain after the local exchange end control block 66 via block 79 has`reset` all counters within poller 71. In the case of the firstembodiment, this condition is reached when the local exchange end has nomore customers which it may validly poll (either because no more cellsare waiting or all customers have exhausted their `d` values).

In the second embodiment each customer is polled until his `d` counterin poller 71 reaches zero, regardless of whether he has packets waiting.When his counter reaches zero, the customer cannot be polled until thepolling of all other customers has been completed. This includes anyallocation control devices which are due to be polled (nb. assuming 32customers, one such control device is due to be polled every 125microseconds). When no further customers can be polled the controller 66via block 79 resets all counters.

Similarly, in the first embodiment during the ranging periods of thetelephony channel, the local exchange end will poll (via block 71) allcustomers who have been active during the last frame. Polling willcontinue to the level of each customer's `d` value regardless of whetherthey have cells waiting or not. Then the local exchange end 66 willreset all its counters and block 71 begin polling again according to thesame pattern.

Whenever the local exchange end performs a reset of the counters, itindicates this to all customers' equipments by setting the control fieldof a downstream cell to indicate `reset`. This information is used byeach customer's equipment to reset different service queues of waitingcells to control the amount of cells which any one service queue cansubmit for upstream transmission.

In the first embodiment, every customer is given an initial `d` value of1, and this guarantee of access is maintained. For cells with aninformation field of 32 octets, a `d` of 1 is equivalent to primary rateaccess to all 32 customers on demand.

In the second embodiment, an idle customer will have a `d` value of zeroto prevent unnecesary polling, but every customer is guaranteed a `d`value of 1 on demand (via blocks 56, 52, 50 and 45 to the exchange endblocks 73, 63 and 66), therefore no customer may increase his guaranteedaccess rate to a level where other customers could be refused a minimum`d` of 1.

The controller 66 at the local exchange may receive requests from block56 to increase a customer's `d` value via the `change allocation`setting. It will comply with this request provided the current level ofguaranteed access is not already equal to the available capacity (i.e.approximately 65 cells every 125 microseconds, given a gross bit rate ofaround 150 Mbit/s).

For example, to determine the correct allocation for the variable bitrate services (data, VBR video), the customers equipment block 56 viablock 60 will measure the number of cell arrivals, say every 4milliseconds. If the number of arrivals exceeds the current `d` (or partof `d` assigned to these services) the customer's equipment as justdescribed will signal to the local exchange end for an increase in its`d` allocation. As already stated, the local exchange end will complywith this request if possible, but will not allow already allocatedbandwidth to be used.

In the case of the first embodiment, change allocation information issupplied whenever the customer is next polled having first indicatedthat a packet is waiting via the telephony channel. Again in the case ofthe first embodiment occasionally the customers' equipment will generatea signalling call to verify its current `d` requirements and ensure thatthe local exchange end remains in step with all recent changes. Thefrequency of these transmissions from all customers' equipments may beused to provide a minimum level of monitoring of the round-trip delay soas to compute any necessary fine tuning to be conveyed to the customers'equipment via the ordinary telephony channel.

To signal to the local exchange end the customer's equipment sends an`increase d` setting on its next upstream control cell.

In the case of the second embodiment, the local exchange end block 66maintains both a copy of each customer's ideal requirements in the dREQregister and also holds the permitted `d` value in another register inblock 66. Normally `d` is set equal to dREQ. However, `d` may be heldless than dREQ until additional capacity becomes free. When thishappens, any customer `d` value which is less than its dREQ can beincreased by 1 via block 76 (and no `d` can be increased by 2 until allother such customers have received an increase of 1).

The delay experienced by a cell will depend upon its service priorityand the allocation of the total `d` value (for a given customer) whichhas been assigned to its service queue. In the case of the firstembodiment, the delay is composed of two parts:

a round-trip delay to indicate the cell arrival to the exchange end andreceive permission to transmit from the exchange end.

a random waiting period. This would be typically less than 10microsecpnds (and would never be more than 125 microseconds for thosecells requiring guaranteed access).

In the case of the second embodiment because the local exchange end doesnot wait for an arriving call indication before polling, the randomcomponent of delay is the only component arising in this case.

Thus the ATD channel is capable if carrying a wide mix of services, somebursty and some requiring guaranteed bandwidth. It may be used in anenvironment where there are some telephony-only customers on theoriginal channel mixed with other customers whose services have migratedonto the ATD channel.

The propose channels would allow the flexible use of bandwidth to carrydialogue services as they evolve. The migration from narrow band tobroadband to the ATD situation could be effected in stages as required.

Although the passive network has been described in terms of opticalfibre, other configurations e.g. copper cable or wire-less transmissioncoulld be envisaged.

Although end 15 of FIG. 3 has been described as the exchange end, thisterm is meant to encompass a location where multiple packet handling isarranged for a number of remote users (sometimes called a head end).

I claim:
 1. A bidirectional broadband and telephony network controllablefrom an end of the network including means (67) at the end of thenetwork for allocating time slots for information packets to travelwithout interference to and from a plurality of receiving stations,wherein the allocating means includes means (70) for determininground-trip delay between the end of the network and each of thereceiving stations and means (70) for adjusting the round-trip delay toensure correct spacing of the information packets during passage throughthe network and includes means (71, 66) for interrogating stations andenforcing allocation thereto to ensure each station's minimum bandwithrequirements is fulfilled, and, means (71) for dynamically allocatingspare capacity to each station for bursty services.
 2. A network asclaimed in claim 1 wherein the means for allocating time slots includingmeans (67) at the end of the network for generating a continuous seriesof cells, means (74) for transmitting said cells, means (61) for routinginformation to be carried to the receiving stations in selected cellsand means (68) for generating information within a header portion of acell to allow a designated station access to transmit an informationpacket using that cell or retain an information packet from that cell.3. A network as claimed in claim 2 wherein the means for generatinginformation includes means (61) for generating station identificationinformation in the cell header indicative of a receiving station andmeans (61) for generating a connection number for receipt by eachstation.
 4. A network as claimed in claim 1, wherein the allocatingmeans includes means (68) for generating control information indicativeof the presence and type of information contained in each cell forreceipt by the receiving stations.
 5. A network as claimed in claim 4wherein the means for generating control information includes means (68)for generating a cell full/empty indicator, means (68, 71) forgenerating a reset/transmit indicator for service access, and means (61,67) for generating a broadcast indicator to allow all customers toretain the received packet.
 6. A network as claimed in claim 4 whereinthe means for generating control information includes means (61, 67) forgenerating a priority indicator.
 7. A network as claimed in claim 4wherein the means for generating control information includes means for(66) for generating a change in allowed allocation.
 8. A network asclaimed in claim 1 wherein each station includes means (56, 52) forrequesting an alteration in its amount of access.
 9. A network asclaimed in claim 1 wherein polling means (71) are provided at the end ofthe network for repeatedly polling each station to determine currentuser requirements.
 10. A network as claimed in claim 9 wherein eachstation includes means (56, 52) for requesting a change in transmissionaccess and the end of the network includes means (66) for responding tothe request to change allocation or to maintain or decrease allocationand generator means (66, 71) are provided for generating information tothe requesting station indicative of any change.
 11. A network asclaimed in claim 10 wherein the requesting means includes detector means(77, 76, 52) for checking the received allocation changes and means (52)for generating a further request if an error is detected.
 12. A networkas claimed in claim 10 wherein the requesting means includes anallocation request store (78), an allocation check store (78) and anallocation modification store (78).
 13. A network as claimed in claim 9wherein the polling means (71) includes counter means (71) fordecrementing the allocation of each station on handling each cell untilits allocation is exhausted and means (66, 71) for resetting the countermeans when no further stations have allocation remaining and/or nofurther cells from or for any station are waiting to be transmitted. 14.A network as claimed in claim 9 wherein the polling means (71) isconfigured to effect access via the telephony channel.
 15. A network asclaimed in claim 1 wherein the means for determining the round tripdelay includes means (70, 67) for transmitting ranging signals during ameasuring period.
 16. A network as claimed in claim 1, wherein the meansfor adjusting the round trip delay includes a phased lock loop (50) ateach station to ensure synchronization and an elastic buffer (50)controllable by the end of the network to ensure an equal number of bitsin each round trip path from any station.
 17. A network as claimed inclaim 1, wherein a telephony channel is provided and interface means(48) are provided thereto to convey information on the transmissionrequirement of a station.
 18. A network as claimed in claim 17 whereinthe interface means includes means (48, 71) for determining packetarrivals at respective stations, and means (71) for polling stations fortheir allocation requirements dependent on information queuing therein.19. A network as claimed in claim 1, wherein information on thetransmission requirements of a station are sent in the header ofinformation packets.
 20. A network as claimed in claim 1, whereinswitching means (42) at the end of the network are provided for handlingincoming and outgoing information on a multiplexed basis.
 21. A networkas claimed in claim 20, wherein the switching means (42) is configuredto handle information on an asynchronous time division basis on a singlecommon wavelength.
 22. A network as claimed in claim 1, wherein eachstation includes wavelength filter means (20) to permit receipt ofbroadband packet information.
 23. A network as claimed in claim 1,wherein each station includes buffer means (50, 53, 54) for temporarilyholding information sent or received and means (52) for arranging saidinformation into the cell format required.
 24. A network as claimed inclaim 1, wherein the network is configured using an optical medium andan optical/electrical interface (45, 46) is provided at each station.25. A network as claimed in claim 24 wherein the end of the networkincludes an optical/electrical interface (73, 74).
 26. A network asclaimed in claim 1, wherein the means (71, 66) for enforcing allocationincludes means (66) for providing at least a minimum access on requestby a station and means (66, 71) for maintaining at least such allocationtill it is no longer required thereby and means (66) for providing atleast a minimum access to further stations to ensure all requestingstations some access regardless of bandwidth constraints.
 27. A networkas claimed in claim 1 including means (61, 64) at the end of the networkfor translating a connection number of an incoming/outgoing packetto/from the associated station.
 28. A network as claimed in claim 1,wherein each station includes means (56) for calculating the accessrequirements from current traffic information to determine if anincreased allocation is required.
 29. A network as claimed in claim 1,wherein cell generator means (64, 63) at an end of the network areconfigured to generate cells in the form of ATD cells complete with aninformation header portion.
 30. A method of controlling a bidirectionalbroadband and telephone network from an end of the network includingallocating at the end of the network time slots for information packetsto travel without interference to and from a plurality of receivingstations, the allocation including determining round-trip delay betweenthe end of the network and each of the receiving stations; adjusting theround-trip delay to ensure correct spacing of the information packetsduring passage through the network; interrogating stations and enforcingallocation thereto to ensure each station's minimum bandwidthrequirements is fulfilled; and, dynamically allocating spare capacity toeach station for bursty services.
 31. A method as claimed in claim 30including generating at the end of the network a continuous series ofcells, transmitting said cells, routing information to be carried to thereceiving stations in selected cells and generating information within aheader portion of a cell to allow a designated station access totransmit an information packet using that cell or retain an informationpacket from that cell.
 32. A method as claimed in claim 31 includinggenerating station identification information in the cell headerindicative of a receiving station and generating a connection number forreceipt by each station.
 33. A method as claimed in claim 30, whereinthe allocation includes generating control information indicative of thepresence and type of information contained in each cell for receipt bythe receiving stations.
 34. A method as claimed in claim 33 wherein thecontrol information generation step includes generating a cellfull/empty indicator, generating a reset/transmit indicator for serviceaccess, and generating a broadcast indicator to allow all customers toretain the received packet.
 35. A method as claimed in claim 33 whereinthe generating control information step includes generating a priorityindicator.
 36. A method as claimed in claim 33, wherein the controlinformation step includes generating a change in allowed allocation. 37.A method as claimed in claim 30 including at any station the step ofrequesting an alteration in its amount of access for transmissiondetermined by the exchange end.
 38. A method as claimed in claim 30including repeatedly polling each station to determine current userrequirements.
 39. A method as claimed in claim 38 including requestingfrom any station a change in transmission access and responding from theend of the network to the request to change allocation or to maintain ordecrease allocation and generating information to the requesting stationindicative of any change.
 40. A method as claimed in claim 39 whereinthe requesting step includes checking the received allocation changesand for generating a further request if an error is detected.
 41. Amethod as claimed in claim 38 wherein the polling step includesdecrementing the allocation of each station on handling each cell untilits allocation is exhausted and reallocating the stations when nofurther stations have allocation remaining and/or no further cells fromor for any station are waiting to be transmitted.
 42. A method asclaimed in claim 30 includes transmitting ranging signals during ameasuring period to determine the round trip delay, and generating asignal to adjust an elastic buffer at any station to equalize the roundtrip delay of all stations.
 43. A method as claimed in claim 30, whereininformation on the transmission requirements of a station are sent inthe header of information packets.
 44. A method as claimed in claim 30,including multiplexing incoming and outgoing information at the end ofthe network on an asynchronous time division basis on a single commonwavelength.
 45. A method a claimed in claim 30 including providing anoptical transmission medium and interfacing electrical signals theretoat both the end of the network and each station.
 46. A method as claimedin claim 30, including providing at least a minimum access on request bya station and maintaining at least such allocation till it is no longerrequired thereby and providing at least a minimum access to furtherstations to ensure all requesting stations some access regardless ofbandwidth constraints.
 47. A method as claimed in claim 30, includingtranslating at the end of the network a connection number of anincoming/outgoing packet to/from the associated station.
 48. A method asclaimed in claim 30, including calculating at each station the accessrequirements from current traffic information to determine if anincreased allocation is required.
 49. A method as claimed in claim 30,including generating cells at the end of the network in the form of ATDcells complete with an information header portion.